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<TITLE>80386 Programmer's Reference Manual -- Opcode MOVS</TITLE>
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Chapter 17 -- 80386 Instruction Set</A><BR>
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<H1>MOVS/MOVSB/MOVSW/MOVSD -- Move Data from String to String</H1>

<PRE>
Opcode  Instruction      Clocks   Description

A4      MOVS m8,m8       7        Move byte [(E)SI] to ES:[(E)DI]
A5      MOVS m16,m16     7        Move word [(E)SI] to ES:[(E)DI]
A5      MOVS m32,m32     7        Move dword [(E)SI] to ES:[(E)DI]
A4      MOVSB            7        Move byte DS:[(E)SI] to ES:[(E)DI]
A5      MOVSW            7        Move word DS:[(E)SI] to ES:[(E)DI]
A5      MOVSD            7        Move dword DS:[(E)SI] to ES:[(E)DI]
</PRE>

<H2>Operation</H2>

<PRE>
IF (instruction = MOVSD) OR (instruction has doubleword operands)
THEN OperandSize := 32;
ELSE OperandSize := 16;
IF AddressSize = 16
THEN use SI for source-index and DI for destination-index;
ELSE (* AddressSize = 32 *)
   use ESI for source-index and EDI for destination-index;
FI;
IF byte type of instruction
THEN
   [destination-index] := [source-index]; (* byte assignment *)
   IF DF = 0 THEN IncDec := 1 ELSE IncDec := -1; FI;
ELSE
   IF OperandSize = 16
   THEN
      [destination-index] := [source-index]; (* word assignment *)
      IF DF = 0 THEN IncDec := 2 ELSE IncDec := -2; FI;
   ELSE (* OperandSize = 32 *)
      [destination-index] := [source-index]; (* doubleword assignment *)
      IF DF = 0 THEN IncDec := 4 ELSE IncDec := -4; FI;
   FI;
FI;
source-index := source-index + IncDec;
destination-index := destination-index + IncDec;
</PRE>

<H2>Description</H2>

MOVS copies the byte or word at [(E)SI] to the byte or word at
ES:[(E)DI]. The destination operand must be addressable from the ES
register; no segment override is possible for the destination. A segment
override can be used for the source operand; the default is DS.
<P>
The addresses of the source and destination are determined solely by the
contents of (E)SI and (E)DI. Load the correct index values into (E)SI
and (E)DI before executing the MOVS instruction. MOVSB, MOVSW,
and MOVSD are synonyms for the byte, word, and doubleword MOVS
instructions.
<P>
After the data is moved, both (E)SI and (E)DI are advanced
automatically. If the direction flag is 0 (<A HREF="CLD.htm">CLD</A> was executed), the registers
are incremented; if the direction flag is 1 (<A HREF="STD.htm">STD</A> was executed), the
registers are decremented. The registers are incremented or decremented by 1
if a byte was moved, 2 if a word was moved, or 4 if a doubleword was moved.
<P>
MOVS can be preceded by the 
<A HREF="REP.htm">REP</A> prefix for block movement of CX
bytes or words. Refer to the 
<A HREF="REP.htm">REP</A> instruction for details of this operation.

<H2>Flags Affected</H2>

None

<H2>Protected Mode Exceptions</H2>

#GP(0) if the result is in a nonwritable segment; #GP(0) for an illegal
memory operand effective address in the CS, DS, ES, FS, or GS
segments; #SS(0) for an illegal address in the SS segment; #PF(fault-code)
for a page fault

<H2>Real Address Mode Exceptions</H2>

Interrupt 13 if any part of the operand would lie outside of the effective
address space from 0 to 0FFFFH

<H2>Virtual 8086 Mode Exceptions</H2>

Same exceptions as in Real Address Mode; #PF(fault-code) for a page
fault
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<B>up:</B> <A HREF="c17.htm">
Chapter 17 -- 80386 Instruction Set</A><BR>
<B>prev:</B><A HREF="MOVRS.htm"> MOV Move to/from Special Registers</A><BR> 
<B>next:</B><A HREF="MOVSX.htm"> MOVSX Move with Sign-Extend</A>
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